A color wheel is often disposed in a projection apparatus. The color wheel includes a motor and a color filter module. The color filter module is disposed on the motor and rotated by the motor. The color filter module includes a red, a green and a blue color filter for filtering three primary colors, and a digital micro-mirror device (DMD) is used together for adjusting the color contrast of projected images.
DMD belongs to a linear response display and therefore a gamma correction is needed for adjusting the color value. However, the gamma correction causes the gray scale to be quantified and then become discontinuous. In prior art, an error diffusion filter is used for solving the above drawback, but meanwhile apparent dither noise is shown in the gray scale of low brightness.
Please refer to FIG. 1. A conventional color filter module 10 includes two green color filters 12 with high transmittance, two red color filters 14 with high transmittance, two blue color filters 16 with high transmittance and further includes a green color filter 18 with low transmittance that is about 6%. Among the green gray scales displayed in the conventional color filter module 10, a green gray scale with high brightness is displayed by the two green color filters 12 with high transmittance; a green gray scale with middle brightness is displayed by the two green color filters 12 with high transmittance and the green color filter 18 with low transmittance; a green gray scale with low brightness is mainly displayed by the green color filter 18 with low transmittance. In such disposition, the problem of dither noise is solved.
Please refer to FIG. 2 that shows the spectrogram of each green color filter 12 with high transmittance and each green color filter 18 with low transmittance. Each green color filter 12 with high transmittance has a first transmittance 20. 50% of the first transmittance 20 is corresponding to a first light wavelength 22 and a second light wavelength 24, wherein the second light wavelength 24 is larger than the first light wavelength 22. The green color filter 18 with low transmittance has a second transmittance 26 that is smaller than the first transmittance 20. 50% of the second transmittance 26 is corresponding to a third light wavelength 28 and a fourth light wavelength 30, wherein the fourth light wavelength 30 is larger than the third light wavelength 28. In the conventional color filter module 10, the gray scale is displayed by two color filters of different transmittances, and the first light wavelength 22 must be equal to the third light wavelength 28 while the second light wavelength 24 must be equal to the fourth light wavelength 30 for preventing the gray scale from variation.
In practice, the coating technique of color filters nowadays for making the first light wavelength 22 equal to the third light wavelength 28 while the second light wavelength 24 equal to the fourth light wavelength 30 has low yield rate, high cost and production difficulty. Therefore, the condition for light wavelengths needs to be improved.